TRANSMIT VERSUS RECEIVE GAIN – PART TWO
We left off our previous discussion of Transmit versus Receive Gain with the question:
Since increasing either transmit or receiver gain has the practical effect of making the image brighter, what difference does it make?
The most obvious distinction is that the transmit signal propagates INTO the patient whereas the receive signal returns FROM the patient. Why this matters relates to the “L” in the ALARA principle.
More than 25 years ago, the American Institute of Ultrasound in Medicine (AIUM) outlined guidelines for the ‘prudent use’ of ultrasound scanning. The guiding principle is that responsible use of diagnostic ultrasound involves conducting studies and assessments that limit exposure and duration.
Therefore, quite clearly the goal for ultrasound operators should be limiting transmit power (gain/voltage/intensity) to “As Low As Reasonably Achievable” to produce accurate clinical results. With that in mind, if an acceptable image can be acquired through the manipulation of receive gain, it would be preferable to an increase in transmit gain.
However, enhancing the receive gain results in a proportionate increase in the ‘noise’ accompanying the return signal. In everyday experience, you can compare this to ‘dialing up’ the volume knob on your car radio to enhance a weak (or distant) broadcast station. While the broadcast does become louder, so do the hisses, pops and cracking sounds.
It is a happy coincidence that this discussion comes immediately after the Thanksgiving holiday, because anybody who traveled by car across state will identify with the experience of a fading radio signal as he or she drew more distant from a larger city (assuming you are using FM and not Satellite Radio). There came a point where no amount of manipulation in volume could improve audibility. In such cases, your only choice was to scan the dial for stronger transmission frequencies to hear.
Obviously, with diagnostic ultrasound, the transducer is the one and only ‘broadcast station’. As the operator, you therefore must make the assessment when strengthening the transmit signal is necessary to improve resolution and accuracy.
Lastly, I would add that by changing the scanning angle, the transmit frequency, the focus, and the view used, you can potentially improve clinical results. So with that in mind, increasing the transmit power may represent your second or third option in achieving quality scans.
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This subject matter is discussed more thoroughly in Frank Miele’s Ultrasound Physics and Instrumentation in Chapter 5: Transducers and Chapter 6: System Operation. Related discussions are in Chapter 3: Attenuation and Chapter 8: Artifacts.
